Papers by Author: Christoph Leyens

Abstract: In this study the oxidation resistance of Ti-45Al-8Nb (at.%) alloy coated with quaternary Ti-Al-Cr-X layers (X = Si, Hf, Y, Zr and W) was investigated. The Ti-Al-Cr-Si, Ti-Al-Cr-Hf and Ti-Al-Cr-Y coated specimens were exposed to air at 950°C under cyclic conditions, whereas the samples with Ti-Al-Cr-Zr and Ti-Al-Cr-W coatings were thermally cycled at 1000°C. After the maximum exposure time period of 1000 1h-cycles or failure cross-sections of the samples were examined by means of SEM and EDS to analyse the microstructural evolution. At initial stages of exposure, all intermetallic layers formed a thin alumina layer on top, providing a diffusion barrier to oxygen. But interdiffusion between coating and substrate caused depletion of the Ti(Cr,Al)2 Laves phase in the intermetallic layers, which promoted the formation of alumina, as well as transformation into Ti-rich B2-phase. Coarsening of the latter phase beneath the alumina scale resulted in a higher oxidation resistance compared to that of ternary Ti-Al-Cr coating.

Abstract: Due to a nanolaminate structure, MAX phases are materials with an interesting set of properties. The present paper is focussed on the synthesis and characterization of Ti2AlC and Ti2AlN MAX phase coatings. They were deposited by dc magnetron sputtering from single elemental Ti, Al, and C targets (Ti-Al-C system); in addition to Ti and Al, nitrogen was used for the Ti-Al-N system. XRD analysis revealed the growth of cubic Ti3AlC and Ti3AlN perovskite phases in the coatings deposited at 540°C. After coating deposition an annealing treatment at 800, 1000 and 1200°C was carried out. The results indicate that annealing for 1 h in vacuum at 800°C enhances crystallization of the Ti2AlN and Ti2AlC MAX phases. It was also observed that annealing at temperatures higher than 1000°C enhances the decomposition of both phases, Ti2AlC and Ti2AlN, and gives rise to the formation of the carbide and nitride phases TiCx and TiNx, respectively.

Abstract: A novel canning technology to forge gamma-TiAl alloys was developed at the BTU Cottbus. A TiAl specimen was encapsulated with multilayer stainless steel foil and glass. The steel foil layers prevented the heat loss through radiation and the glass layer reduced the temperature decrease through conduction. First, the effect of steel foil on the cooling rate was investigated. Cooling curves were recorded for TiAl specimens without steel foil layer, with 1, 2, 3 and 4 layers of steel foil, as well as with 3 coated steel foil layers, respectively. While the unprotected specimen cooled from 1200 °C to 1100 °C within 12 s, the specimen with 3 coated steel foil layers needed 52 s for the same temperature decrease. The efficiency of the glass layer was examined with forging of steel specimens. The cooling rate during forging of the specimen with a glass layer was only half of that without a glass layer. Based on the results, Ti-45Al-0.5Mo-0.5Cu-0.2Si specimens, canned with steel foil and glass, were successfully forged at strain rates of 0.1 s-1 and 0.04 s-1 with warm dies which were heated to 500 °C. Visual and metallographic examinations revealed no cracks, pores or micropores. The microstructures are fine-equiaxed grains.